1. Field of the Invention
The present invention relates generally to a method and an apparatus for transmitting a high speed packet data in a CDMA mobile communication system, and more particularly to a method and an apparatus for differentiated power distribution in high speed packet transmission.
2. Description of the Related Art
Like other kinds of channels, channels for high speed packet transport employ channelization codes in order to maintain their independence. However, from among channelization codes being used, the channelization codes used by the channels for high speed packet transport utilize shorter codes, that is, codes having smaller spreading factors (for example, spreading factors of not higher than 32), than those for the other channels. Further, in high speed packet transport, a high order modulation scheme is employed in order to increase data transport speed of the entire system. Therefore, the high speed packet transport employs link adaptation techniques, which are different from those in 2nd generation radio communication systems employing a fixed modulation scheme.
In general, mobile communication systems employ power control technologies to effectively use radio resources. 2nd or 3rd generation mobile communication systems employ especially fast power control technologies. Mobile communication systems for high speed packet transport employ an Adaptive Modulation and Coding Scheme (hereinafter, referred to as “AMCS”) for effective assignment of radio resources, in contrast to 2nd generation mobile communication systems employing a fixed coding rate and modulation scheme.
Power control technology implies a technology of controlling transport power of a node B or each User Equipment (hereinafter, referred to as “UE”), enabling all UEs to receive uniform service from the same node B. In other words, the power control enables a UE having a relatively bad channel condition to use a transport power higher than a transport power used by a UE having a relatively good channel condition, so that signals transmitted from all UEs can reach a node B with uniform power level. The node B enables all UEs to receive signals with uniform power by individually determining the transport power of the signals in consideration of the channel condition of each UE. In this case, the channel condition may depend on a distance between the node B and each UE. For example, the UE having a relatively bad channel condition may be a UE located far from the node B, and the UE having a relatively good channel condition may be a UE located adjacent to the node B.
Next, AMCS is a technology of modifying the coding rate and modulation scheme of a UE as a downlink condition changes. In the AMCS, each UE periodically checks the downlink condition and reports Channel Quality Information (hereinafter, referred to as “CQI”) determined from the checking, to the node B. The node B estimates the downlink condition for the corresponding UE by means of the CQI, and determines the proper coding rate and modulation scheme for the corresponding UE on the basis of the estimated downlink condition. The determination of coding rate and modulation scheme is usually carried out by Modulation and Coding Scheme (hereinafter, referred to as “MCS”) levels, which are determined by the CQI. For high speed packet transport, High Speed Downlink Packet Access (hereinafter, referred to as “HSDPA”) and 1X-EVDV have been proposed until recently. In the HSDPA and 1X-EVDV, modulation schemes of QPSK, 8PSK, 16QAM, and 64QAM are being discussed and coding rates of ½ and ¾ are being considered for AMCS. Therefore, in a system employing the AMCS, relatively high-order modulation schemes of 16QAM and 64QAM and a relatively high coding rate of ¾ are applied to UEs using good quality channels, such as UEs located adjacent to a node B. In contrast, relatively low-order modulation schemes of 8PSK and QPSK and a relatively low coding rate of ½ are applied to UEs using bad quality channels, such as UEs located at boundary regions between cells.
The two technologies described above, which are the power control and the AMCS, are different types of link adaptation techniques and are independently applied to a mobile communication system.
FIG. 2 is a block diagram illustrating a conventional transmission apparatus employing an AMCS for high speed packet transport. Referring to FIG. 2, a controller 210 receives information about a downlink condition from each UE and determines a modulation scheme and a coding rate for each UE on the basis of the information, so that data to be transmitted to the corresponding UE can be encoded and modulated by means of the determined modulation scheme and coding rate. In other words, the better the downlink condition is, the higher-order the modulation scheme and the higher the coding rate are, as assigned by the controller 210. In contrast, the worse the downlink condition is, the lower-order the modulation scheme and the lower the coding rate are, as assigned by the controller 210. Coding and modulation units 220 and 230 receive corresponding user data, encode and modulate the user data by means of the coding rate and modulation scheme determined by the controller 210, and then output modulation symbol arrays according to UEs. Then, the modulation symbol arrays are input to a demultiplexer (DEMUX) 240. Under the control of the controller 210, the demultiplexer 240 distributes the modulation symbol arrays corresponding to the number of channels, which will be used. That is, the demultiplexer 240 separates the output of the coding and modulation units 220 and 230 according to the number of codes assigned to each UE. In the embodiment illustrated in FIG. 2, M number of modulation symbol arrays according to UEs are distributed to n number of channels. In this case, it is preferred that n, representing the number of channels, is larger than or equal to M, representing the number of UEs or users. The modulation symbol arrays, which are divided according to the channels, are inputted to corresponding spreaders 250-1, 250-2, . . . , and 250-n, and are spread into corresponding channelization codes by the corresponding spreaders 250-1, 250-2, . . . , and 250-n. The assignment of codes to the respective UEs is carried out according to a determination of an upper scheduler or a code assignment controller (not shown). The signals having been spread with the N channelization codes W1 to Wn are added in an adder 260 that thus outputs one signal array. The signal array output from the adder 260 is gain-compensated with power having a usable magnitude in a power gain unit 270 and is then outputted. In conclusion, in the conventional transmission apparatus, power is uniformly distributed according to the same gain compensation for each channel.
When employing the AMCS described above, the magnitudes of assigned powers are different according to MCS levels. This difference will be described hereinafter with reference to FIG. 3, which illustrates an example of coding rates and modulation schemes determined by MCS levels of UEs. In this case, the MCS levels are classified into four stages.
Referring to FIG. 3, each MCS level has a predetermined assignment range corresponding to the channel condition, which has an upper critical point and a lower critical point. For example, reference numeral 301 in FIG. 3 designates an upper critical point of an MCS level having a modulation scheme of 64QAM and a coding rate of ¾, and reference numeral 303 designates a lower critical point of the MCS level (in fact, the upper critical point need not be considered when the 64QAM is the highest-order modulation scheme). Of course, when the MCS level having the modulation scheme of 16QAM and the coding rate of ¾ in FIG. 3 is taken into consideration, the reference numeral 303 designates an upper critical point of the MCS level.
Since each MCS level has a specific modulation scheme and a specific coding rate, a modulation scheme and a coding rate which will be applied to a specific UE are determined by the MCS level. That is, the higher the order of the modulation scheme and the coding rate are, the higher the MCS level is. In contrast, the lower the order of the modulation scheme and the coding rate are, the lower the MCS level is. The MCS level is determined by a channel condition, i.e., a Signal to Noise Ratio (hereinafter, referred to as “SNR”). Therefore, the best channel condition, that is, the best SNR is required to employ the highest-order modulation scheme and the highest coding rate corresponding to the uppermost MCS level from among the MCS levels. In FIG. 3, UE #1 310 and UE #2 320 to which the modulation scheme of 64QAM and the coding rate of ¾ are applied require the best SNR. That is, a modulation scheme and a coding rate corresponding to each MCS level are applied to UEs, which can meet the requirement of each MCS level. When the MCS levels have been determined, powers are assigned according to the determined MCS levels. Therefore, powers having the same magnitude are assigned to UEs (or channels), which have been determined as having the same MCS level, even though they require powers of different magnitudes. In this case, the power for high speed packet transport, which can be assigned to the UEs, has a value obtained by subtracting a power arranged for voice service from the total power, which can be used by a node B. The assigned power is used as a parameter for controlling the channel condition. That is, the channel condition corresponding to a predetermined channel can be improved by increasing the power assigned to the predetermined channel. In contrast, the channel condition corresponding to a predetermined channel can be degraded by decreasing the power assigned to the predetermined channel.
In applying the AMCS as described above, the MCS level for each UE is determined according to the channel condition. In the case of the HSDPA being currently discussed, the MCS levels are classified into stages from at least four to at most eight. The AMCS in which the modulation schemes and the coding rates are different according to the channel conditions can be applied best when one channelization code is assigned for a fixed duration. However, the high speed packet transmission system employing the AMCS described above use both code division and time division in order to simultaneously support service for many UEs. This means that a plurality of channelization codes may be assigned instead of a single channelization code. The number of assignable channelization codes plays an important role in determining the number of UEs capable of simultaneously accessing. This implies that multiple UEs can simultaneously receive packets through different channelization codes. In this case, one or more channelization codes may be assigned to one UE for a fixed duration. When a plurality of channelization codes are assigned for a fixed duration as described above, the AMCS cannot effectively use the resources. That is, although the MCS level is determined corresponding to the channel condition and each of different channelization codes, an optimum channel condition cannot be employed due to the limited MCS levels.
In more detail, each MCS level has an upper critical point and a lower critical point, and the same MCS level is given to UEs having channel conditions between the two critical points. Therefore, even the UEs (UE#1 and UE#2 in FIG. 3) having channel conditions with a relatively large difference may be provided with the same MCS level, and even the UEs (UE#2 and UE#3 in FIG. 3) having channel conditions with a relatively small difference may be provided with different MCS levels. For example, referring to FIG. 3, the UE#2 320 having a channel condition slightly exceeding a lower critical point 303 and the UE#1 310 having a channel condition adjacent to an upper critical point 301 are provided with the same MCS level, although they have considerably different channel conditions. In contrast, referring again to FIG. 3, the UE#2 320 having a channel condition slightly exceeding a lower critical point 303 and the UE#3 330 having a channel condition adjacent to the lower critical point 303 are provided with different MCS levels, although they have approximate channel conditions. Therefore, the UE#2 320 provided with the same MCS level as that of the UE#1 310 has a higher probability of generating errors in comparison with the UE#l 310, so that the UE#l 310 has a higher probability of requiring retransmission. Further, the UE#2 320 provided with an MCS level higher than that of the UE#3 330 has a higher probability of generating errors in comparison with the UE#3 330.
In the conventional transmission apparatus employing the AMCS, since each MCS level has too broad a range, as described above, the same power may be assigned even to UEs having channel conditions which are much different from each other, thereby deteriorating the transport power. Moreover, different MCS levels may be provided to UEs having channel conditions, which are only slightly different from each other. This problem can be overcome by adjusting powers assigned to UEs provided with MCS levels. That is, surplus powers of UEs having more assigned powers than needed can be redistributed to UEs having assigned powers insufficient for the services required by the corresponding MCS levels provided with the assigned powers, so as to solve this problem.